1. Field of the Invention
The present invention relates to a developing apparatus using toner which is a colorant for developing an electrostatic latent image formed on an image bearing member into a visible image, more particularly, to a developing apparatus using a one-component developer as the toner.
2. Description of the Related Art
Electrophotographic image forming apparatuses, such as copiers, printers and the like, include a developing apparatus which is adapted to form an electrostatic latent image on a photosensitive member serving to bear the latent images, to supply a developer, such as toner which is a colorant, and to make the toner deposit selectively, thereby making the latent image visible.
In the above-mentioned developing apparatus, a latent image formed on the photosensitive member is developed into a toner image which is transferred to a transfer medium such as a shoot. After the image transfer, a part of the toner that was not transferred to the sheet remains on a surface of the photosensitive member. The remaining toner is removed from the surface of the photosensitive member so that the photosensitive member is prepared for the subsequent image forming operation. To this end, a cleaner unit is provided for removing the residual toner from the photosensitive member surface after transfer operation. The toner thus removed by the cleaner unit is received by a toner chamber in the cleaner unit.
Image forming apparatuses with a developing apparatus arranged above mentioned have been faced with a demand for downsizing. To meet this demand, reduced are spaces for various processing means which are located around the photosensitive member and involved in the image forming process. The reduced spaces for the processing means result in a further demand for downsizing the respective processing means themselves. In this connection, a strong demand exists for a compact developing apparatus.
Specifically, as to the developing apparatus, a developing roller of a magnetic brush type is provided which utilizes a magnetic force for supplying a two-component developer composed of toner particles and a magnetic carrier to a developing region in opposing relation with the photosensitive member, and every time the development operation completes, a residual developer is recovered to a developing tank. Hence, control is provided for stable developing performance such that toner of consumed amount may be made up thereby to maintain a constant concentration of toner or content ratio of the toner in the developer.
Generally, the developing apparatus of the above-mentioned type, namely of the magnetic brush type tends to have a large overall size because the developer contains a greater proportion of carrier and requires a large developing tank for storage of the developer. In addition to the need for controlling the toner concentration, a stirring member or the like is needed for maintaining a constant charge of the toner particles in the developer. A plurality of stirring members provided in the developing apparatus constitutes a bottleneck in the downsizing of the developing apparatus.
On the other hand, a developing apparatus using a one-component developer or carrier-free toner has been proposed and put to practical use. Such a one-component toner developing apparatus permits the size reduction thereof because the control of toner concentration is not necessary and the absence of carriers permits the reduction of capacity of the developing tank. In addition, such a developing apparatus features easy maintenance. More specifically, the developing apparatus eliminates the need for a maintenance work for replacing a deteriorated developer particularly resulting from deteriorated carriers.
Further, the developing apparatus only needs the replenishment of toner, obviating the necessity of sensing the toner concentration. Inasmuch as control for sensing the toner concentration is not necessary, simple control may be provided. Particularly, the developing apparatus using the one-component toner only needs the replenishment of toner when required.
In the developing apparatus utilizing one-component toner, the toner must be supplied to and made to deposit on the developing roller. In the case the single-component toner is magnetic toner, the developing roller imparted with a magnetic force is capable of magnetically attracting a desired amount of toner thereby forming thereon a toner layer of a uniform thickness. However, in order to impart the magnetic force to the developing roller, the developing roller need to be constructed such that a plurality of magnets are disposed along a direction of rotation while an outer periphery of the arranged magnets are covered with a non-magnetic cylindrical sleeve. As a result, the apparatus tends to be increased in size.
Compared to this, in the case of a developing apparatus utilizing non-magnetic one-component toner, the developing roller is formed of an elastic material such as rubber, not relying on the magnetic force for attracting the toner. Accordingly, the developing roller may have a simple structure and a reduced diameter, thus permitting the weight reduction. However, since the developing roller is not adapted to magnetically attract the toner, an important point is to bring a constant amount of toner or a uniform toner layer into contact with the photosensitive member.
To this end, a feed roller is provided with respect to the developing roller, while a blade is provided so as to be pressed against the developing roller for regulating the toner mass per unit area, thereby maintaining the thickness of fed toner constant. The blade is typically configured so that a face of a sheet-like member constituting the blade is press contacted at a suitable pressure. The amount of toner supplied to the developing roller is regulated by suitably pressing the surface or belly of the sheet-like blade member against the developing roller whereby a uniform toner layer is formed on an overall axial area of the developing roller.
An alternative method wherein an end or edge of the blade rather than the face thereof is pressed against the roller has been proposed in Japanese Examined Patent Publication JP-B2 60-15068(1985).
In these known blade structures, when the blade is pressed against the developing roller at its face of belly, is wherein the face or belly of the blade is pressed against the developing roller, the toner fusion may be advantageously prevented. Unfortunately, however, when toner of a good fluidity is employed for good toner feeding to the developing roller, the blade needs to be pressed against the roller at an increased pressure for accomplishing an optimum toner mass per unit area for the developing process. This results in an increased driving torque of the developing roller, thus requiring a large driving motor.
Although a uniform toner layer of a very small thickness may be formed by pressing the tip end or edge of the blade against the roller, a sufficient amount of toner for the developing process may not be achieved.
The regulation of the toner mass per unit area may be accomplished by adjusting the setting position of the blade under the condition including both conditions of pressing the surface or belly of the blade against the roller and pressing the tip end or edge of the blade against the roller. This permits formation of a suitable toner layer which is constant in thickness and sufficient in quantity for performing the developing process under suitable conditions. Unfortunately, however, this arrangement provides little allowance for the setting position of the blade, resulting in a very difficult adjustment of the setting position of the blade. Furthermore, serious variations in the toner mass per unit area depositing on the roller are caused by wear or the like of the blade so that increased frequencies of the blade replacement result. In addition, each blade replacement involves a very cumbersome adjustment work, thus lowering the serviceability.
Aside from the aforementioned blades, there is known a blade in which an end portion of the blade is shaped like "L" in section such that a bent thereof is pressed against the developing roller. This blade offers a benefit that the amount of toner may be regulated under an intermediate condition between the aforesaid conditions of pressing the end or edge of the blade and of pressing the belly or face of the blade.
Unfortunately, when this blade is pressed against the roller at its bent, a curvature of the bent thereof is varied to cause significant variations in the toner mass per unit area. Hence, the bent must be formed with an extremely high precision. This means a demanding work for fabricating the blade.
A method for stabilizing the toner mass per unit area on the developing roller has been disclosed in Japanese Unexamined Patent Publications JP-A 7-64391(1995) and JP-A 7-239611(1995). According to this method, a tip end of the blade is slightly angled for attaining a desired toner mass per unit area. This method contributes to an increased allowance for the setting position of the blade, thus negating the need for the exacting adjustment work.
The developing apparatus disclosed in Japanese Unexamined Patent Publication JP-A 7-64391(1995) or the like provides an increased allowance for the setting position of the blade serving to maintain a constant amount of toner which is deposited on the developing roller and ensures stable toner feeding.
In actual apparatuses set forth in the above publication, however, an angle of aperture of a toner inlet portion defined by the blade and the developing roller is varied due to deformation of the developing roller or difference in diameter of the developing rollers. This may lead to serious variations in the toner mass per unit area deposited on the developing roller, although the blade is set to position according to predetermined conditions. The deformation of developing roller means that the developer roller is elastically deformed by pressing the blade against the roller. The difference in the diameters of the developing rollers is attributable to variations in the size of respective rollers generated in fabrications or difference in the type of the developing apparatuses.
Accordingly, if an allowance for setting is insufficient because of problems regarding the angle of aperture defined by the tilt angle of the slant face formed in the blade or the bending angle of the blade is reduced due to the aforementioned causes, then the cumbersome adjustment work becomes necessary. That is, a need exists for reworking the end portion of the blade according to each developing roller thereby to provide a sufficient allowance for the setting position of the blade. In other words, a specific blade having a slant face conforming to each developing roller must be fabricated. This leads to cumbersome operations for fabricating the blades which are required of high precision.
The angle of aperture of the toner inlet portion defined by the developing roller and the blade is not taught in Japanese Unexamined Patent Publications JP-A 7-64391(1995) and JP-A 7-239611(1995).
There is another problem regarding a case where the blade is mechanically bent for forming the slant face thereof. That is, distortion due to residual stress caused by the bending of the blade impairs the straightness of the blade so that the blade becomes incapable of forming the uniform toner layer along the axial direction of the developing roller. In a case where the blade is formed of a rolled material, the distortion is further increased due to the residual stress produced in the rolling process so that the straightness of the blade is impaired similarly to the aforesaid case. As a result, the blade becomes incapable of forming the uniform toner layer along the axial direction of the developing roller.
In the generally known blades formed of a metal sheet, a metal material having a good resilience, such as phosphor bronze, stainless steel and the like, is used rather than aluminum and the like which has a relatively poor resilience. The blade formed of such a material suffers the toner fusion to the surface thereof if it is used over an extended period of time. The toner fused to the blade causes variations in the thickness of the toner layer with respect to the axial direction of the developing roller, and production of partial streaks results. If the developing roller in this state is used for developing the latent image, a degraded image and the like result.